GDP can be calculated at home with the following
equation:
y = c + i + g + n
where...
C = consumption: the sum of all goods and services purchased, related to GDP (people will spend no matter what, but they will spend more if they have more)
I = investment: the sum of all private investment in the country, including construction and surplus inventory, proportional to GDP (it only happens when people have extra resources)
G = government spending: the sum of all government contracts, exogenous to GDP (the government will spend no matter what)
N = net exports: the sum of all exports minus the sum of all imports, inversely related to GDP (as GDP goes up, the country brings in more and sends out less)
A related figure is
aggregate expenditures, which is almost the same as GDP, but doesn't count surplus inventory. Another related figure is
natural GDP, the highest GDP attainable before
inflation starts to increase.